Age-related macular degeneration (AMD) is one of the leading causes of irreversible vision loss in the western world accounting for 75% of legal blindness of the population of age 50 and older in developed countries (1). The prevalence of AMD which is of 0.05% before 50 years old, rises to 11.8% after 80 years of age and is expected to double in the coming decades because of the projected increase in aging populations (2, 3).
The causes of AMD are poorly understood, but it is agreed that the progressive decline of vision in AMD results from the dysfunction of the central retina principally its underlying elements, the retinal pigment epithelium (RPE), the Bruch membrane (BM), the choriocapillaris and degeneration of the photoreceptors (4). Other than age, few predisposing factors have been clearly identified; these include light, cigarette smoking, possibly hypertension and atherosclerosis (5). In this context, despite their specific characteristics an analogy between deposits found in AMD and atherosclerosis has been proposed (6).
Early AMD is characterized by focal or diffuse sub-RPE debris in BM (Drusen and basal deposits respectively), changes in RPE pigmentation and by thinning and obliteration of the choriocapillary layer (4). Two clinical forms of late AMD are identified: the non-exudative form characterized by geographic atrophy of RPE and choroid (geographic atrophy, GA) and the exudative form, which also includes choroidal neovascularisation (CNV) (7). Although the non-exudative form is disabling due to patchy defects in the central visual field, it is the choroidal neovascularisation of the exudative form that leads to blindness via its leaky vessels that prone to subretinal exsudations and hemorraghes (1) leading to the destruction of macular photoreceptors. The clinical features common for both types of AMD include the deposition of amorphous white deposits of phospholipids and oxidized lipoproteins (drusen), and inflammatory mediators that develop between the RPE and the BM as well as the hypo/hyperpigmentation of the RPE.
Although the underlying pathogenesis and its sequence that leads to AMD is not yet defined, the key pathophysiological steps have been summarized as 1) impaired transport between the RPE and the choriocapillaris leads to debris accumulation in the interposed BM, 2) deposition of drusen leads to RPE and photoreceptor degeneration, and 3) deregulation of the balance of pro- and anti-angiogenic factors leading to choroidal involution or neovascularisation (CNV). As CNV is a key factor in preserving vision in the aged population, the development of therapeutic agents that impairs CNV has been considered for the treatment of AMD (8).
The development of CNV in AMD has been thought to be induced by the hypoxia due to the reduced diffusion of oxygen and nutrients from the choroid to the retinal pigment epithelium (RPE) following the thickening of Bruchs membrane resulting from the deposit of lipid and protein material (9). This hypoxia conjugated with the choroid hypoperfusion induces a significant upregulation of the expression of VEGFs and VEGFRs in the RPE cells as well as in the endothelial cells of the choriocapillaris (10, 11) promoting therefore angiogenesis in age related macular degeneration.
The treatment strategies in AMD are mainly targeted to inhibit the ocular neovascularisation by blocking the expression or the activity of VEGFs and its receptors.
The blocking the expression of VEGF and its receptor has been approached by the silencing RNA technology. Silencing VEGF using SiRNA technique has been proposed by Acuity Pharmaceuticals in the development of Cand 5 (12). Intravitreal injection of Cand 5 was found to inhibit the neovascular growth without systemic toxicity.
The same approach of the siRNA technique to downregulate the expression of VEGFR-1 following the intravitreal and periocular injections of Sirna-027 has been proposed by SIRNA therapeutics (13). It was found effective in reducing choroidal and retinal neovascularisation (14). However, the long-term effect of SiRNA approaches remains to be documented.
The most frequent antiangiogenesis approach in the treatment of AMD consists of the inhibition of VEGF binding using specific aptamer, anti-VEGF antibodies or sVEGFRs.
The first development of aptamer (Pegaptanib), a covalent conjugate of an oligonucleotide and PEG that binds to the extracellular isoforms of VEGF was initiated for the treatment of neovascular AMD. Although this innovative approach appears highly promising, it does not reduce the CNV development and is unable to improve overall vision (15).
The anti VEGF therapy using the recombinant humanized Fab derived from the anti-VEGF murine monoclonal antibody (Ranibizumab) or the full-length anti-VEGF monoclonal humanised antibody (Bevacizumab) has been reported to be effective in preventing the formation of CNV (16) with a significant decrease in central retinal thickness. Although, the therapy using antibodies against VEGF which inhibits all VEGF isoforms has a drawback since VEGF is also a survival factor for neuronal cells and a fundamental requisite for the maintenance of the fenestration of the choriocapillaris which is necessary for the physiological function of the choroid itself, the retinal pigment epithelium and the outer retina. The chronic inhibition of VEGF could lead to the atrophy of these tissues.
The development of a fusion protein featuring a higher binding affinity to VEGF which combines extracellular domains of VEGFR-1 and 2 to The Fc portion on IgG1 (VEGF-TRAP) has been shown to inhibit CNV following its systemic administration. However, the adverse effect of hypertensive crisis following systemic administration of this ligand prevented further exploration in the treatment of AMD (17).
Blocking VEGF activity by interfering with its signalling pathways has been explored. Effectively, VEGF binding to its receptors leads to the phosphorylation of cytoplasmic signalling proteins such as PI3 kinase, MAP kinase and PKC. The selective inhibition of isoforms of PKC by SU 5416 (18) (a VEGF inhibitor) or PKC412 (19) reduces CNV development with less angiographic leakage. However, systemic adverse reactions such as nausea and hepatic toxicity have been reported.
The inhibition of the cellular effect of VEGF with the use of intravitreal steroids has been considered for the treatment of neovascular AMD and exudative retinal diseases. Triamcinolone acetonide has been shown to feature angiostatic effect in animal models with CNV (20). The combination of intravitreal steroid treatment with photodynamic therapy appears to give better vision outcomes (21). However, the major disadvantages of such treatment consist of the rise of intraocular pressure with the progression of cataracts (22). The new generation of modified steroids (cortesines) such as Anecortave acetate, which is devoid of glucocorticoid and mineralocorticoid activities responsible for the steroid-associated adverse effects is in evaluation for the prevention of AMD development (23).
Thus, there is still a need to develop antiangiogenic strategies to stop the neovascular growth and leakage in the treatment of AMD. Recent reports have shown that the accumulation of oxidized lipoproteins in the RPE cells and Bruchs membrane, which is consistent with the accumulation of cholesterol esters and phospholipid-containing debris in the Bruchs membrane, is paralleled with that of macrophages in the AMD lesions (24). The macrophages express scavenger receptors and may accumulate for the uptake of oxidized lipoproteins. Suppressing the macrophage accumulation by controlling macrophage responses to oxidative lipoproteins and phospholipid oxidation might be complementary for the treatment of AMD (24).
Among the seven families known of scavenger receptors, CD36a type B scavenger receptor has been shown to be involved in multiple functions: (1) cellular energy uptake as a long chain fatty acid (LCFA) receptor (25), (2) clearance of oxidized low density lipoprotein (oxLDL) (26), (3) phagocytosis of retinal outer segments (ROS) for the recycling of spent photoreceptor disks (27), (4) mediation of the antiangiogenic effect of thrombospondin-1 (28). Interestingly, CD36 was found expressed in RPE, microvascular endothelial cells and in microglia (29) which are major cell types in AMD as well as in macrophages found in CNV membranes (24). Its expression could be upregulated by oxLDL and by other oxidative and oxidation-prone products including docosahexaenoic acids a predominant fatty acid in retinal tissue particularly the outer segment (30). As the oxidation process increases with age, oxidized lipoproteins are internalized for subsequent degradation by these cells. A deficiency in the clearance of these oxidized lipids as observed in the LDL-R null or ApoE null mice (31, 32) resulted in the accumulation of debris (drusen) in subRPE and BM. The localization of CD36, its scavenging function towards oxidized lipids and its modulatory role in angiogenesis, makes this receptor an interesting potential candidate for the genesis of AMD by way of lipid build up in BM, retinal degeneration, and vascular obliteration resulting ultimately in the development of neovascularisation.
Growth hormone-releasing peptides (GHRPs) consist of a family of small synthetic peptides derived from enkephalins that were developed as growth hormone secretagogues (33). These peptides feature high affinity binding to the ghrelin receptor (GHS-R1a) a G-coupled receptor mainly expressed in the hypothalamus and are involved in the stimulation of growth hormone-release (34). Besides their endocrine activity, GHRPs feature GH-independent cardioprotective activity in improving post-ischemic cardiac dysfunction (35) and antiatheroscierotic activity, preventing the development of atherosclerotic plaques in the ApoE null mice model (36). This beneficial effect appears to be CD36-dependent and might be due, at least in part, to the reduction of the oxLDL uptake by macrophages and to the increase of cholesterol and phospholipid efflux from macrophages through the activation of transcription factors PPARγ and LXRα and the ABC transporters (37). The peripheral activity of GHRPs might be mediated by their interaction with the scavenger receptor CD36 as shown by covalent photolabelling study with a photoactivatable derivative of hexarelin, the hexapeptide prototype of GHRPs (38) which binds also with high affinity to the GHS-R1a receptor (34).